Activities of marine product industries have been restricted because of problems of environment contamination, overfishing and territorial waters. The catches of fish have become small and it has been difficult to keep fishing grounds.
In view of the above-mentioned problem, cultivation fishery has been spreaded. Namely, effective production of fishes and shells is planned by surrounding a part of a sea area such as a calm gulf, a lake or a river having good conditions with nets and by supplying foods and drugs. On the other hand, however, such cultivation fishery causes a problem of environment destruction. Namely, the waste of foods and excrement discharged by fishes accumulate on the bottom of the sea and contaminate the sea area.
In view of these problems, there has been practiced a method wherein cultivation fishery is conducted by supplying breeding water in a breeding tank installed on land. According to this method, the problem of environment contamination can be avoided by the installation of a waste water purification apparatus. Further, rational cultivation can be obtained by using various industrial operations.
In an attempt to further develop this idea, there has been noted a method of breeding fish or shells in a closed system. In the closed system, a highly powerful water purifying is needed wherein the amount of waste water can be reduced as much as possible by minimizing the replacement of breeding water. If the above-mentioned technique is practically used, the problem of environmental contamination can be eliminated and a stable supply of fresh marine products can be attained without creating a problem for land and other places. Further, the temperature of a breeding tank can be adjusted in an economical manner. Accordingly, the kinds of fish to be cultivated can be increased and production can be increased, whereby development of a new fishery can be expected.
There has been used a breeding water purifying apparatus in a closed breeding system wherein conventional technology of water treatment such as a sand filtering device is used. A breeding water purifying apparatus with a sterilization device is known. In this case, sterilization is conducted by using a chlorine-type reagent or ultraviolet rays or ozone (which are relatively new methods).
FIG. 8 is a diagram showing an embodiment of a conventional breeding apparatus used in an aquarium, which is disclosed in "Cultivation of Marine Products and water", Second collection (Scientice Corp., 1987, p 111). In FIG. 8, reference numeral 31 designates a breeding water tank, numeral 32 designates a circulating pump for breeding water, numeral 33 designates a sand filtering device, numeral 34 designates a blower and numeral 35 designates an air scattering device.
In FIG. 8, breeding water of reduced quality due to the excrement of fishes to be cultivated and foods left is supplied from the breeding water tank 31 to the sand filtering device 33 by means of the circulating pump 32. In the sand filtering device 33, the breeding water is purified by removing floating solids and by removing organic substances and contaminating components dissolved in water such as ammonical nitrogen by the action of microorganisms on the surface of sand. The purified breeding water is returned to the breeding water tank 31. The blower 34 is for supplying air which contains oxygen necessary for the existence and the growth of fish. Air supplied from the blower 34 is scattered in the breeding water by means of the air scattering device 35 to thereby supply oxygen in the breeding water. When substances difficult to purify increase in the breeding water and the quality of the breeding water becomes deteriorated so that it is unsuitable for the breeding, fresh water is supplied to the system, or the contaminated water is replaced by fresh water.
In the conventional breeding apparatus, sterilization by a chlorine-type reagent is usually conducted before the water is fed into the breeding water tank. As described before, ultraviolet rays or ozone may be used instead of a chlorine-type reagent.
In the conventional circulation type breeding apparatus with a purifying function as described above, there have been requirements wherein as great an amount of foods should be given to fishes so that high density cultivation be attained. However, a closed-type breeding apparatus capable of satisfying such requirements could not be practically used. The major reason resides in the problem of becoming with infected disease in the fish to be cultivated.
Namely, since the conventional breeding apparatus has no sterilization means as shown in FIG. 8, the fish are infected with disease when germs enter into the breeding water tank by any cause. Even though a chlorine-type reagent is used for sterilization to prevent the fish from becoming infected with disease, the reagent is apt to live in water and is toxic to the fish to be cultivated. Accordingly, there is a restriction as to the amount of administration, and it is sometimes difficult to obtain sufficient sterilization.
Sterilization by ultraviolet rays has an advantage of resulting in no residual toxicity. However, when the surface of an ultraviolet ray lamp is stained, the effect of sterilization is reduced because of the reduction of transmittance of the ultraviolet rays whereby the quality of breeding water becomes deteriorated. Accordingly, the use of ultraviolet rays has problems of reliability and the maintenance when a breeding apparatus is practically used.
Ozone the has advantages of high sterilizing effect and less residual property (decomposition into oxygen) in comparison with the chlorine-type reagent. Further, it has an oxidative destruction effect to contaminant components unlike the chlorine-type reagent. Accordingly, ozone has a desired nature of purification in a closed type breeding apparatus. However, when the breeding water contains sea water, ozone reacts with bromine ions existing in the sea water whereby residual oxidizing products having toxicity is produced. As a result, the same problem as the chlorine-type reagent arises.
In studying the entirety of the above-mentioned problems, the sterilization and purification functions obtained by ozone in the closed-type breeding apparatus is still attractive, and if the problem of the residual oxidizing product in the case where sea water is used for breeding water is dissolved, the possibility of realizing a closed type breeding system can be increased. Even though the residual oxidizing product can be easily removed by the addition of reducing agent, it is not practical from the viewpoint of stability and maintenance.